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Los modelos de precisión en los datos fotométicos:
G, BP y RP
J.M. Carrasco (+UB team)
III Reunión Científica de la REG, Sitges 23-25 enero 2013
Outline
• Gaia photometry & spectrophotometry
• Photometric performances website
• Photometric performances in GOG
GAIA PHOTOMETRY & SPECTROPHOTOMETRY
Figures courtesy EADS-Astrium
Blue photometer:330–680 nm3-27 nm/pixel
Red photometer:640–1050 nm7-15 nm/pixel
of fused silica
BP/RP spectrophotometers
BP/RP spectrophotometry
K
K
eff
eff
GOG
BaSeL-2.2 stars library
BP/RP spectrophotometry
G: white‐light, Astrometric field.
GBP and GRP: integrated from low resol. spectra
GRVS: integrated from RVS high resol. spectra
Gaia passbands
PHOTOMETRIC PERFORMANCES
WEBSITE
Includes all known instrumental effects + 20% science marginThis is the single‐field‐of‐view transit, taking into account all CCDs along scan
G Photometric standard errors per transit
Gaia Science Performance website
Includes all known instrumental effects + 20% science margin
XP Photometric standard errors per transit
Gaia Science Performance website
XP Photometric standard errors per transit
Gaia Science Performance website
• division of the single‐field‐of‐view‐transit photometric standard errors by the square root of the number of observations (~70 in average).• With an assumed calibration error of 30 mmag at CCD‐level
Units: mmag
End-of-mission errorsGaia Science Performance website
PHOTOMETRIC PERFORMANCES
IN GOG
GOG spectra Epoch Spectra: single transit
– BP/RP: reduced into a common wavelength scale– RVS: each spectrum has its wavelength scale
Combined Spectra: the end‐of‐mission– BP/RP/RVS completely reduced into a common wavelength scale
Sigma spectrum: standard deviation Gaussian noise model– For each GOG output spectrum (epoch and combined)
GOG spectra are obtained by randomly sampling the sigma spectrum to generate a noise vector which is added to the noise‐free spectra – User can choose if the output spectra is noise‐free or observed
GOG: from input to observable spectra
1. Convolve input with rotational (vsini) profile (RVS only)2. Normalize to source magnitude and extinction3. Apply instrument transmission4. Apply PSFs convolution following dispersion relation5. Addition of constant background and saturation effects6. Transform pixels into samples (to 1D)7. Apply GOG photometric error modeling sample to sample8. Associate a wavelength to each sample9. Produce sigma spectrum associated to the (noise‐free)
sample spectrum (both for epoch and combined spectra)10. Generates observed spectrum
Sartoretti & Isasi (2011), Jordi et al. (2010)
GOG: Photometric error per CCD-transit
X magnitude error computed taking into account:
• the photon noise (ns samples within the window)
• the total detection noise per sample r, which includes the detector read‐out noise
• the sky background contribution bX assumed to be derived from nb background samples
• fx: object flux within a given passband X
• gaper: factor accounting for the light loss produced because of the finite extent of the window (=1 by default, configurable)
• m : overall additional error margin (m=1.2, configurable)• nstrips : : the number of CCD strips in each band (9 in G, 1 in BP/RP and 3 in RVS)• σcal : the contribution of the calibration error per transit (=30 mmag, configurable)
Jordi et al. 2010, A&A 523, 48
=0 in these plots
m=1.2 (20% margin)
Jordi et al. 2010, A&A 523, 48
These values will determine
Gaiavariability detection
capabilitiesG=0.02 mag at
G=20
BP-RP=0.5 at G=20 for V-I=1
cal relevant if G↓↓
GOG: Photometric error per CCD-transit
Single transit uncertainty:
Number of observations (Nobs) after 5 years Detection Probability (DP) in G and XP
Jordi et al. 2010, A&A 523, 48
End of mission uncertainties:
TBD in GOG
GOG: Photometric error at end-of-mission
GOG: XP spectra error model
Same procedure as for the photometry, now applied to each sample:
• Poisson noise in the source and background
• Total read‐out noise
• Background‐subtraction noise
• Calibration noise
• 20% error margin added to accommodate unmodelled random errors (charge transfer inefficiency and imperfect geometric calibration)
• Systematic errors are currently neglected
End-of-mission SNR per band
cal = 10 mmag, 80 transits, unreddenend stars (Carrasco et al. 2006)
Thank you!!